Abstracts

Rationale: Recent work using microelectrodes, which have a diameter of <100 µm, has suggested that clinically relevant information about the ictal onset range may be found by recording from microdomains composed of areas as limited as a single microelectrode. However, this experimental work was performed using specialized, high impedance acquisition systems designed for this purpose, and it remained uncertain if these findings would also be seen using standard clinical acquisition systems with microelectrodes. Methods: Intracranial EEG recording using macroelectrodes and microelectrodes was performed in a 47 year old gentleman who had seizures for 14 years following a severe car accident with traumatic brain injury. He was having 1-3 seizures a month on 2 medications and had only ever experienced 2 consecutive months of seizure freedom. The recording was acquired using a XLTEK EMU 128FS amplifier using Neuroworks software, sampling at 500 Hz. Two Ad-Tech hybrid electrodes (6 macrocontacts, 10 microcontacts) were used, with the microcontacts located at the tip of the depth electrode and between the 4 deepest macroconatcts. One of these depth electrodes was placed with the deepest contacts in the head of the right amygdala and the second with the deepest contacts in the right hippocampus. In addition, 76 additional macrocontacts were used including 2 right subtemporal strips, a right orbitofrontal strips, and a 64 contact grid over the lateral right frontotemporal neocortex. Results: During monitoring, the most frequent epileptiform activity were spikes over the deepest macro-contacts in the depth electrodes in the amygdala and in the hippocampus, which were often associated with spikes in the micro-contacts. Two of the patient's habitual seizures were recorded with onsets in the mesial temporal region starting in the amygdala depth and posterior subtemporal strip. When medications were tapered, discharges were noted that were confined to a single micro-contact on the depth electrode in the amygdala (electrode AD12) and were not apparent on any of the neighboring macroelectrodes or other microelectrodes. These included microseizures characterized by 15-20 Hz 200-250 µV fast activity which evolved into 350-450 µV 4-7 Hz periodic sharp waves, which lasted 20-40 seconds and could occur up to 20 times an hour. The patient went on to have a right amygdalohippocampectomy and anterior temporal lobectomy and seizure freedom off medications. Conclusions: This is an example of how the presence of microseizures may indicate a potential ictal focus utilizing microcoelectrodes with a standard clinical EEG acquisition system during intracranial recording. With careful analysis microelectrodes may provide additional data to the clinician, which is not available using standard macroelectrodes. However further research must be done to better understand the significance of these findings and how the limitations of clinical EEG amplifiers impact intracranial recordings from microelectrodes.